The bis-(thioacetoxy) aurate (I), or Au(TA)2, and other histochemical and biochemical methods for acetylcholinesterase (AChE) and pseudocholinesterase (pseudo-ChE) will be applied to (1) continuation of a survey of the localization of the enzymes in certain critical tissues, (2) investigation of the sites of synthesis of pseudo-ChE in the superior cervical ganglion (SCG) of the cat, (3) investigation of the neurotrophic or other mechanisms responsible for the maintenance of AChE in the SCG of the cat, (4) testing the hypothesis that pseudo-ChE functions as a precursor of AChE, and (5) identification of the ultrastructural sites of anterograde and retrograde axonal transport of macromolecules. Exploration will be initiated for the development of electron microscopic methods for the localization of (1) the high affinity choline uptake system of cholinergic neurons, (2) the ATPase system that provides the energy for axonal transport, (3) nicotinic cholinergic receptors, and (4) acetylcholine.